This invention relates to methods of forming superparamagnetic magnetite colloidal nanocrystal clusters and construction of colloidal photonic crystals using these clusters as building blocks.
Recent advances in colloidal synthesis have enabled the preparation of high quality nanocrystals with controlled size and shape. Focus of synthetic' efforts appear to be shifting to creation of secondary structures of nanocrystals, either by self-assembly or through direct solution growth. Manipulation of the secondary structures of nanocrystals is desired in order to combine the ability to harness the size-dependent properties of individual nanocrystals with the possibility to tune collective properties due to the interactions between the subunits.
Superparamagnetic nanocrystals have proved to be very promising for biomedical applications as they are not subject to strong magnetic interactions in dispersion. Iron oxide nanocrystals have received the most attention for this purpose because of their biocompatibility and stability in physiological conditions. Several robust approaches have been developed for synthesizing magnetic iron oxide (e.g., γ-Fe2O3 or Fe3O4) nanocrystals with tightly controlled size distribution, typically through organometallic processes at elevated temperatures in non-polar solvents. Additional steps of surface modification or lipid encapsulation are usually performed to transfer the hydrophobic nanocrystals from non-polar solvent to water for biomedical applications. The nanocrystals prepared using these methods, with dimensions of order ten nanometers (nm), have a low magnetization per particle so that it is difficult to effectively separate them from solution or control their movement in blood using moderate magnetic fields, thus limiting their usage in some practical applications such as separation and targeted delivery. Increasing the particle size increases the saturation magnetization, but also induces the superparamagnetic-ferromagnetic transition (at a particle size ˜30 nm for Fe3O4) so that nanocrystals are no longer dispersible in solution. The strategy of forming clusters of magnetite nanocrystals has the advantage of increasing the magnetization in a controllable manner while retaining the superparamagnetic characteristics.
Accordingly, what has been needed and heretofore unavailable are superpara-magnetic nanocrystals that overcome the deficiencies of existing configurations so as to eliminate the problem of increasing the particle size producing nanocrystals that are not dispersible in solution. The present invention disclosed herein satisfies these and other needs.
Besides magnetic separation, these magnetite colloidal nanocrystal clusters also find its application in construct novel solution form photonic crystals. Photonic crystals are spatially periodic dielectric structures displaying photonic bandgaps in which certain optical modes can not exist. They have attracted much attention because of their important optoelectronic applications where manipulation of photons is required, for example, as photonic components intended for telecommunications, lasers, and sensors. Among these applications, a highly desirable feature is to have a tunable bandgap, which can be conveniently controlled by external stimuli. Although considerable efforts have been devoted along this direction by changing the refractive indices of the materials, the lattice constants or spatial symmetry of the crystals, the tunability has been typically limited to tens of nanometers in diffraction wavelength. A known system was developed from fabricated colloidal photonic crystals using charged polystyrene microspheres containing superparamagnetic nanoparticles. Changes in diffraction wavelength above one hundred nanometers can be achieved by imposition of magnetic fields.